Na+/I- symporter (NIS) is an intrinsic membrane glycoprotein that mediates active iodide uptake into thyroid follicular cells. We and others have shown that NIS gene transfer can induce iodide uptake into a variety of cells and that xenografts expressing exogenous NIS could be imaged in vivo. These data suggest that NIS expression and function could be measured quantitatively in vivo based on NIS' ability to uptake radioiodide. Thus, NIS may serve as an imaging reporter gene ("lmagene") to optimize vector delivery, to monitor therapeutic gene expression, and to map the tissue/organ sites of repopulated progenitor cells in vivo. The proposed project aims to address the two parameters that are vital to the usefulness of NIS as an Imagene: acceptable signal-to-noise ratio and conditions in which a linear relationship exists between gene expression and measured signal levels. We hypothesize that these two parameters will vary, based upon target cells and in vivo host environment. We will determine the relative importance of NIS expression levels and the number of NIS expressing cells required for in vivo imaging in tissues of different signal to noise ratios. Four specific aims are identified: Aim 1. Determine the range of NIS expression levels that correlate with radioiodide accumulation in cultured cells; Aim 2. Determine the NIS expression levels and the number of NIS expressing cells required for in vivo imaging in subcutaneous xenografts; Aim 3. Investigate the different NIS expression levels required for in vivo imaging in internal organs with different signal to noise ratios; and Aim 4. Evaluate the feasibility of NIS as an imaging reporter gene to monitor therapeutic gene expression in a preclinical animal model. The success of the proposed studies will allow us to proceed to human clinical trials using NIS as an Imagene to ensure targeted and effective gene transfer and to track repopulated progenitor cells in vivo.